Radiation sources for electromagnetic waves have been increasing in number with continued advances in industrial technology and the general standard of living. The leakage of electromagnetic waves, therefore, has become a significant social problem since the wave can exert adverse influences on the human body. Such leakage may also cause the incorrect behavior of integrated circuits in electronic equipment. In particular, electromagnetic waves radiated from electronic computers and various office equipment exert advarse influences on television sets and audio equipment.
Even in the car industry, electronic devices are now used. For example, they are used in automatic controllers for various machines such as an engine, and further, in a speedmeter, a tachometer, and so forth. Several types of cars are now equipped with a microcomputer. In addition, electronic devices such as a telephone, a radio and a TV set are installed inside cars for the convenience of the driver. However, these electronic devices are liable to suffer from various operational difficulties under the influence of electromagnetic waves emanating from the engine compartment, or from a source outside the car.
The most serious problems in an automobile are troubles caused by transient phenomena that occur when the current fluctuates as machines in the car are operated, by induction at the route of load, by capacitive coupling, by inductive interference from a dynamo, by transient voltage due to radiation from the ignition system, and by electromagnetic waves emitted from the radio equipment. In particular, the wires connecting various parts in the engine compartment are under high voltage, and therefore they emit intensive electromagnetic waves which adversely affect the closely packed speed controllers, exhaust gas controller, engine controllers, etc. This makes it necessary to shield power lines, signal lines, control lines, and other lines in the engine compartment.
Electromagnetic waves are also radiated from electric wires and cables. They enter electronic machines and equipment directly or through an antenna, causing misoperation of IC's and LSI's which operate with extremely small currents. The conventional electric wires and cables are of such a structure that the insulation layer is covered with an electrically-conductive compound filled with carbon black so that power loss is minimized. The carbon blackfilled layer, however, does not prevent the radiation of electromagnetic waves.
Various methods have recently been used to shield such equipment against electromagnetic waves in British Pat. Nos. 1,073,567, 1,168,701, 1,195,076, 1,226,181 and 1,246,829 and U.S. Pat. Nos. 3,468,753, 3,491,056, 3,393,257, 3,474,067, 3,668,281, 3,673,121, 3,767,613, 4,006,114, 4,234,469 and 4,367,306.
In general, since metals can absorb or reflect electromagnetic waves, they are effectively used as shielding materials for electromagnetic waves radiated from an electronic oven and various communication apparatuses. In addition, plastics with metals deposited thereon by vacuum metallizing method or arc spray coating method can be used for the same purpose. Furthermore, materials prepared by adding additives, e.g., carbon powder and metal powder, to plastics can be used.
Using metals alone or other materials coated with metals are disadvantageous in that the specific gravity is large, moldability is poor, the procedure of treatment is not simple, and the treatment cost is high. Similarly, a method of introducing additives is disadvantageous in that when these additives are added in small amounts, the effect cannot be exhibited sufficiently. However, when they are added in large amounts, the mechanical strength and moldability of the resulting molded article are seriously reduced although the effect can be exhibited.